The present invention relates to a method of manufacturing a fuel assembly applicable for a core of a nuclear reactor using a fuel containing Pu.sup.239, members constituting the fuel assembly (sometimes referred to herein as "fuel assembly elements"), and alloys used for the members. In particular, the present invention concerns a method of manufacturing a fuel assembly applicable for a reactor core in which a water-uranium fuel volume ratio is 1.5 or less and the conversion ratio from U.sup.238 to Pu.sup.239 is high, members constituting the fuel assembly, and alloys used for the members.
As for members constituting a fuel assembly used for nuclear power generation, those for a light water reactor use a zirconium alloy; and those for a fast breeder reactor use a stainless steel. A high conversion reactor acts as a bridge between a light water reactor and a fast breeder reactor, and has a feature of effectively converting non-fissionable U.sup.238 contained in natural uranium to fissionable Pu.sup.239 usable for power generation. The non-fissionable U.sup.238, which has been not used in a light water reactor, can be used by the high conversion reactor, resulting in the effective utilization of uranium resource. The stored Pu.sup.239 can be effectively used as a fuel for a fast breeder reactor, or a fuel for a high conversion reactor and a general breeder reactor.
In a conventional light water reactor and a high conversion reactor, the reduction in the exhaust amount of a spent fuel by increasing an operation cycle and the burn-up of fuel contributes to an economic merit, for example in reducing a power generation cost. However, when the operation cycle is increased and the burn-up of fuel is enhanced, the staying period of a fuel assembly in a reactor is increased. This further accelerates the corrosion of the surfaces of members constituting the fuel assembly in water at a high temperature/high pressure. Moreover, the effective conversion from U.sup.238 to fissionable Pu.sup.239 is mainly due to resonance neutrons having an energy higher than that of thermal neutrons. As a result, neutron spectrum in a reactor core is hardened (a large number of neutrons having high energy exist), thus accelerating the damage of the material due to neutrons.
A further problem is that the zirconium alloy (normally used as a high corrosion resisting alloy) has a tendency to become brittle by fast neutron irradiation.
Further, in the environment of a BWR (Boiling Water Reactor), a member constituting a ZIRCALOY fuel assembly generates a local oxidization called the nodular corrosion, and the corrosion portion propagates with time. A method of reducing this corrosion has been known, wherein a heat-treatment of heating a zirconium alloy for a short period of time in a temperature range of (.alpha.+.beta.) phase or .alpha. phase and quenching the alloy is inserted in the downstream step in a member manufacturing process (for example, Unexamined Japanese Patent Publications Nos. SHO 51-110411 and SHO 51-110412, and Examined Japanese Patent Publications Nos. SHO 60-59983 and SHO 63-31543). This known technique is called (.alpha.+.beta.) quenching or .beta. quenching, which is applied to alloys used for the existing light water reactor: ZIRCALOY-2 (Sn: 1.2-1.7 wt %, Fe: 0.10-1.20 wt %, Cr: 0.05-0.15 wt %, Ni: 0.03-0.08 wt %, O: 0.06-0.14 wt %, and the balance: Zr); and ZIRCALOY-4 (Sn: 1.2-1.7 wt %, Fe: 0.15-1.24 wt %, Cr: 0.05-0.15 wt %, O: 0.06-0.14 wt %, and the balance: Zr). Of the above alloy components, Fe, Cr and Ni are elements for improving corrosion resistance, and Sn is an element of improving strength. Fe, Cr, Ni precipitate as intermetallic compounds within crystal grains and crystal boundaries. These intermetallic compounds are refined by the (.alpha.+.beta.) quenching or .alpha. quenching; and further when the cooling rate is sufficiently large, they are dissolved in solid even in the matrix. The mechanism of enhancing the corrosion resistance is not fully understood, but it is generally considered that the refining of precipitations and the increase in the concentration of solid-solution of Fe, Ni, and Cr contribute to the increase in the corrosion resistance.
The improvement of the alloy composition and alloy components leads to the enhancement of the corrosion resistance. Various improved alloys have been known as follows: an alloy improved in corrosion resistance which has the same composition of that of the existing ZIRCALOY but is optimized in the added amounts of the alloy elements (Unexamined Japanese Patent Publication No. SHO 62-228442); an alloy having the composition of ZIRCALOY which is further added with the fifth element such as Nb, Mo, W, V, Te, Ta, Si, Ru, Rh, Pd, Pt, or An (Unexamined Japanese Patent Publication Nos. SHO 60-36640, SHO 63-33535, SHO 64-73037, SHO 64-73038, and HEI 1-242747); an alloy having the composition of Zr--Nb alloy which is further added with elements of Sn, Mo, Cr, Ni, Fe, V, W, and Cu in a slight amount (Unexamined Japanese Patent Publication Nos. SHO 50-148213, SHO 51-134404, SHO 61-170552, SHO 62-207835, and HEI 1-119650); a Zr--Bi alloy (Unexamined Japanese Patent Publication No. SHO 63-290234); and a Zr--Sn--Te, Mo alloy (Unexamined Japanese Patent Publication No. SHO 63-290233). These zirconium alloys are intended to be used for a light water reactor, and thereby they are difficult to be used as they are for a high conversion type future reactor in which neutron spectrum is shifted on a high energy side as compared with the existing light water reactor.
As described above, in the high conversion type future reactor, non-fissionable U.sup.238 is effectively convened into fissionable Pu.sup.239 and is used for power generation. The nuclear transformation is generated by allowing resonance neutrons (energy: 10.sup.0 to 10.sup.4 eV) to absorb U.sup.238. In such a reactor core, it is required to lower a water-uranium fuel ratio and to shift neutron spectrum on a high energy side (spectrum is hardened). As a result, the damage ratio of a member constituting a fuel assembly due to neutrons is increased. Accordingly, to significantly increase the burn-up of a light water reactor and to realize a high conversion type future reactor, it becomes important to improve the neutron damage resistance and the corrosion resistance of a member constituting a fuel assembly and to reduce the capture amount of neutrons of the member.
An object of the present invention, therefore, is to provide a Zr alloy for use with a fuel assembly element, which has a high neutron damage resistance and a high corrosion resistance, and further has a small resonance neutron capture cross-section. Another object of the present invention is to provide a method of manufacturing a member such as a fuel sheath tube constituting a fuel assembly usable for a high conversion type future reactor which is capable of keeping an excellent performance for a long period of time.